Mixed phases having the composition Bi2-x Crx O3 and a process for their production

ABSTRACT

Yellow to orange-red bismuth oxide-chromium oxide mixed phase pigments having a specific surface from about 1 to 10 m 2  /g and the composition Bi 2-x  Cr x  O 3  wherein x is from about 0.05 to 0.5 are produced by mixing the appropriate quantities of finely disperse bismuth-(III)- and chromium-(III)- or chromium-(VI)-compounds, calcining the mixture in air at a temperature from about 500° to 800° C. and subsequently grinding. The products are suitable for pigmenting lacquers, plastics and dispersion dyes.

This invention relates to yellow to orange-red bismuth oxide/chromiumoxide mixed phases having the composition Bi_(2-x) Cr_(x) O₃(0.05≦x≦0.5) with specific surfaces of from 1.0 to 10 m² /g, and to aprocess for their production in pigment quality.

Pure bismuth trioxide (Bi₂ O₃) exists in four different modifications.Of these, only the α-phase is stable at room temperature, δ-Bi₂ O₃ isobtained as the high-temperature phase at temperatures in the range offrom 717° to 825° C. while β-Bi₂ O₃ and γ-Bi₂ O₃ are meta-stable(Gmelins Handbuch der anorganischen Chemie, Vol. 19, page 629 (1964) 8thEdition).

After cooling, the tetragonal β-phase and the cubic γ-phase can bestabilized at room temperature by the addition of metal oxides, such asfor example Sb₂ O₃, Al₂ O₃, Ga_(2A) O₃, In₂ O₃, Y₂ O₃, Fe₂ O₃, Cr₂ O₃,ZrO₂, SiO₂ or GeO₂, to the Bi₂ O₃ -melt (G. Gattow. H. Schroder; Z.anorg. allg. Chem. 318 (1962), page 176).

DE-AS No. 1592388 describes the mixed phase formation of Bi₂ O₃ with aplurality of 1- to 6-valent metal oxides by the calcination ofhomogeneous mixtures of corresponding components, the variousmodifications of Bi₂ O₃ being obtained according to the chemicalcomposition. Although the β-phase of Bi₂ O₃, is stabilized by theincorporation of Cr₂ O₃, which is tomato-red in color, these products donot have the necessary particle sizes to be able to be used as pigments.

An object of the present invention is to enable brilliant bismuthoxide/chromium oxide mixed phases having the composition Bi_(2-x) Cr_(x)O₃ to be obtained in pigment quality by a simple and economic process.

It has now surprisingly been found that bismuth oxide/chromium oxidemixed phases can be obtained in pigment quality providing they have aCr₂ O₃ -content of from 0.8 to 9.7% by weight.

Accordingly, the present invention relates to bismuth oxide/chromiumoxide mixed phases having the composition Bi_(2-x) Cr_(x) O₃ withspecific surfaces of from 1.0 to 10 m² /g and values for x of from 0.05to 0.5.

Particularly favorable colorimetric data are shown by pigments accordingto the invention which have specific surfaces of from 1.5 to 8 m² /g.The BET-surface is determined by the N₂ -method in accordance with DIN661 131.

The present invention also relates to a process for the production ofthe pigments according to the invention which is characterized in thatappropriate quantities of finely disperse bismuth-(III)- andchromium-(III)- or chromium-(VI)-compounds are mixed, calcined in air attemperatures in one step in the range of from 500° to 800° C. andsubsequently ground.

It is surprising that yellow to orange-red mixed phase pigmentscharacterized by high color purity are obtained by this simple process.

Suitable bismuth components are bismuth hydroxide and basic bismuthcarbonate which can be prepared, according to the prior art, byprecipitating bismuth-III-nitrate solutions with alkali metal hydroxidesand/or alkali metal carbonates. In this process however the problem of ahigh content of salts in the effluent (alkali metal nitrates) has to beovercome.

Surprisingly oxidic bismuth compounds which are excellently suitable forthe synthesis of pigments can be obtained without the formation ofsoluble salts when metallic bismuth powder is intensively dispersed inan aqueous suspension at temperatures between 20° and 100° C. with knownstirring apparatuses such as bead mills, dissolvers or Kotthoff mixingsirens. Suitable bismuth metal powders are either obtained by mechanicalcomminution of bismuth bars in ball mills or mortar mills or by sprayinga bismuth melt into water, it being possible for the portion with thepreferred grainsize of <100 μm to be separated off by screening.Depending on the particle size of the metal powder and the height of thetemperature the conversion into the bismuth/oxygen compounds can takedifferent lengths of time. Gassing with air, air/oxygen mixtures and/oroxygen accelerates this process in the same way as the use offinely-divided bismuth powder.

Bismuth oxide (Bi₂ O₃), which has been calcined only once, yieldspigments of considerably poorer pigment quality is less suitable as thebismuth raw material.

Suitable chromium sources are Cr₂ O₃ (BET-surfaces >2.5 m² /g) CrO₃,CrOOH (BET-surfaces >5 m² /g), Cr-(III)-acetate (BET-surfaces >5 m² /g)and precipitated Cr-(III)-hydroxides (BET-surfaces >10 m² /g).

The mixing of the respective components can be carried out both in thedry state and in the moist state, in mixing apparatuses such as, forexample, intensive mixers, mills kneaders, screws or stirrer-equippedvessels.

If oxidic bismuth compounds, which are obtainable by intensivelydispersing finely-divided bismuth metal powder in an aqueous suspension,are used as starting materials for the preparation of the raw-materialmixture the Cr compounds required can be added to the same dispersingapparatus during or after the bismuth metal conversion. This is followedby filtration and drying of the raw material mixture.

A further method of preparing homogenous raw-material mixtures in theco-precipitation of bismuth-III-hydroxides and chromium-III hydroxidesin the required quantitative ratios by the common precipitation of abismuth-(III)-nitrate solution and chromium-(III)-salt solution (forexample Cr-(III)-nitrate or Cr-(III)-acetate in HNO₃) with alkali metalhydroxides or alkali metal carbonates at pH-values above 7.0. Theproduct obtained after filtration is washed free from nitrate, resultingin the formation of homogeneous raw-material pastes having solidscontents of from 30 to 50%.

The moist pastes obtained may be dried by means of spray dryers, dryingcabinets, cylinder dryers and belt dryers. Size-reduction to homogeneouspowder mixtures (residual moisture content up to about 5% by weight) maybe carried out in pinned-disc mills, ball mills, bowl mills or similarunits.

Granulate or pellets may be produced from the moist raw-material mixtureduring drying by means of screws, cylinder dryers or appropriategranulating machines (residual moisture content up to 30% by weight).

The moist paste may also be directly used for calcination, in which caseits moisture content should not exceed 35% by weight for economicreasons.

Calcination of the raw-material mixtures (powder, granulate or moistpaste) may be carried out in air in directly or indirectly heatedfurnaces with or without circulation of the product. Suitable calciningunits are, for example, chamber furnaces, muffle furnaces, rotating drumfurnaces, rotating tube furnaces or swivel hearth furnaces.

After calcination, the products are subjected to preliminary drysize-reduction (for example in mortar mills) and then to wet grinding invibrating sand or ball mills (grinding time between 5 minutes and 3hours). After wet grinding, the product is filtered, washed, dried at105° C. and deagglomerated in a mill.

The particle sizes of the bismuth oxide/chromium oxide mixed phasepigments are largely controllable through the intensity of thecalcination temperature (500°-750° C.). The products obtained havespecific surfaces of from 1.0 to 10 m² /g (as determined by theBET-method in accordance with DIN 661 131).

All the mixed phases are pure according to X-ray photography and havethe structure of β-Bi₂ O₃ (Examples 1 to 12).

The reaction of homogeneous mixtures of Bi₂ O₃ and CrO₃ or Cr₂ O₃ attemperatures of 750° C., involving calcination only once in a rotatingtube furnace, gives distinctly poorer end products (brown-orange colors,in some cases dirty) than the process according to the invention whichis clear proof of the advance over the process described in DE-OS No.1592388.

The orange-red to yellow bismuth oxide/chromium oxide mixed phasepigments with specific surfaces of preferably from 1.5 to 8 m² /gproduced in accordance with the invention are suitable for pigmentinglacquers, plastics and dispersion dyes.

The invention is illustrated by the following examples:

EXAMPLE 1

222.39 g of powdered metallic bismuth were dissolved in 450 ml of 64.5%nitric acid and the resulting solution was adjusted to a pH-value ofapproximately 1.5 by the addition of 1.5 liters of water at roomtemperature, followed by the addition with stirring of 10.91 g ofCr(NO₃)₃.9H₂ O (5-liter stirrer equipped vessel). Precipitation wascarried out with 50% KOH at room temperature up to a pH-value ofapproximately 10. After homogenization for 30 minutes (mixing siren),the product was filtered and washed free from nitrate and the lightgreen paste obtained was dried at 105° C. and size-reduced in a ballmill.

Quantities of one-eighth of the raw-material mixture were calcined inair for 1 hour in a muffle furnace at 500° C., 575° C. and 725° C.Five-eights of this mixture were calcined for 1 hour at 650° C. in thesame way as described above. All the calcines obtained were ground for10 minutes in a mortar mill, their colors varying from yellow-orange(calcination at 500° C.) to red-orange (calcination at 725° C.).

Chemical analysis showed that the products had the composition Bi₁.95Cr₀.05 O₃ (0.8±0.05% Cr₂ O₃ ; 99.0±0.1% Bi₂ O₃).

The batch calcined at 650° C. was subsequently wet-ground with varyingintensity, the following results being obtained:

    ______________________________________                                                                     BET-surface                                      Grinding        Color        m.sup.2 /g                                       ______________________________________                                        10 mins. Wedag mill                                                                           orange-red   1.0                                              10 mins. Wedag mill +                                                                         orange       1.4                                              1 h vibrating ball mill                                                       (wet)                                                                         10 mins. Wedag mill +                                                                         orange       1.7                                              3 h vibrating ball mill                                                       (wet)                                                                         10 mins. Wedag mill +                                                                         Yellow-orange                                                                              3.9                                              1 h sand mill                                                                 (wet)                                                                         ______________________________________                                    

EXAMPLE 2

212.05 g of powdered metallic bismuth were dissolved in 425 ml of 64.5%nitric acid and the resulting solution was adjusted to a pH-value of 1.5by the addition of 1.5 liters of water at room temperature, followed bythe addition with stirring of 71.65 g of Cr(NO₃)₃.9H₂ O (5-literstirrer-equipped vessel). The precipitation and further processing ofthe hydroxide paste were carried out in the same way as in Example 1.

The powder-form raw-material mixture (residual moisture <5%) was dividedinto quarters which were then calcined in air for 1 hour in a mufflefurnace at temperatures of 500°, 575°, 650° and 725° C., respectively.

After the calcines had been ground in a mortar mill (10 minutes), themixed phases were stirred in water for 30 minutes at room temperature(<20% solids contents), filtered, washed, dried and deagglomerated.

According to chemical analysis, the products had the composition Bi₁.7Cr₀.3 O₃ (5.30±0.15% Cr₂ O₃ ; 94.60±0.2% Bi₂ O₃). The dependence of thecolor and the BET-surface on the calcination temperature is shown in thefollowing Table:

    ______________________________________                                        Calcination               BET-surface                                         temperature   Color       m.sup.2 /g                                          ______________________________________                                        500           yellow-orange                                                                             4.5                                                 575           light orange                                                                              3.1                                                 650           orange      1.6                                                 725           orange-red  1.0                                                 ______________________________________                                    

EXAMPLE 3

508.90 g of powdered metallic bismuth were dissolved in 1 liter of 64.5%nitric acid and the resulting solution was adjusted to a pH-value of 1.5by the addition of 3.5 liters of water at room temperature, followed bythe addition with stirring of 171.90 g of Cr(NO₃)₃.9H₂ O (10-literstirrer-equipped vessel). The precipitation and further processing ofthe hydroxide paste was carried out in the same way as in Example 1.

The powder-form raw material mixture (residual moisture content <3%) wasdivided into two halves which were calcined in a muffle furnance for 1hour at 575° and 725° C., respectively, and then ground with varyingintensity as shown in the following table. After filtration, washing anddrying, chemical analysis showed that the products had the compositionBi₁.7 Cr₀.3 O₃ (5.35±0.15% of Cr₂ O₃ ; 94.50%±0.3 of Bi₂ O₃).

The color assessment and specific surfaces of the differently calcinedand differently ground mixed-phase pigments are also shown in the table:

    ______________________________________                                        Calcination                          BET-                                     tempera-                             surface                                  ture °C.                                                                       Grinding         Color       m.sup.2 /g                               ______________________________________                                        575     30 mins mortar mill            6.2                                             5 mins mortar mill            6.2                                             1 h vibrating ball mill                                                       5 mins mortar mill  yellow-orange                                                                           5.6                                             3 h vibrating ball mill                                                       5 mins mortar mill            7.2                                             1 h sand mill                                                        725     30 mins mortar mill  orange-red                                                                              1.0                                             5 mins mortar mill  orange    1.5                                             1 h vibrating ball mill                                                       5 mins mortar mill  light orange                                                                            2.7                                             3 h vibrating ball mill                                                       5 mins mortar mill  yellow-orange                                                                           5.8                                             1 h sand mill                                                        ______________________________________                                    

EXAMPLE 4

In a 10-liter stirrer-equipped vessel, 94.01 g of Cr-(III)-acetate(29.1% of Cr₂ O₃) are dissolved in 400 ml of 65.8% HNO₃ and 3.1 litersof water are added to the resulting solution, followed by theintroduction with stirring of 1006.96 g of Bi(NO₃)₃.5H₂ O (roomtemperature). The solution had a pH-value of 1.5 and was adjusted to apH-value of approximately 8 by the addition over a period of 15 minutesat room temperature of a 31.5% KOH-solution. A green suspension wasformed and was stirred for 30 minutes at pH˜8, followed by furtherprocessing in the same way as in Example 1.

The powdered raw-material mixture was calcined in air both in a mufflefurnace and also in a rotary kiln (in each case for 2 hours at 750° C.)and the calcines obtained were ground for 5 minutes in a sand millwithout preliminary size reduction.

After filtration, washing and drying at 105° C., orange to orange-redmixed-phase pigments are obtained with the composition Bi₁.7 Cr₀.3 O₃(5.30±0.15% of Cr₂ O₃, 94.50±0.1% of Bi₂ O₃) and with BET-surfaces of1.2±0.1 m² /g.

EXAMPLE 5

355.3 g of powdered metallic bismuth were dissolved with 677 ml of ˜60%HNO₃ in a 5-liter stirrer-equipped vessel (temperature increase toapproximately 55° C.), after which the clear Bi-(III)-nitrate solutionwas adjusted to a pH-value of 7 by the addition over a period of 60minutes of an approximately 30.5% KOH-solution. The white suspension ofBi-hydroxide was stirred for another 30 minutes, the mother liquorremoved by decantation and 29.27 g of Cr₂ O₃ (in the form of a 77.8%filter paste) were mixed in over a period of 10 minutes using a Kotthoffmixing siren. The suspension was filtered, washed free from nitrate and,after drying, size-reduced to form a homogeneous powder.

Calcination of the raw-material mixture was carried out in air at 750°C. in a rotating tube furnace and lasted 2 h and 16 h, respectively(gradual heating of the mixture from room temperature to the finaltemperature). The calcines were wet-ground for 5 minutes in a sand millwithout preliminary size-reduction, filtered, washed, dried at 105° C.and deagglomerated.

The end products obtained were orange mixed-phase pigments having thecomposition Bi₁.7 Cr₀.3 O₃ (5.25±0.15% of Cr₂ O₃ ; 94.60±0.2% of Bi₂O₃). The BET-surfaces measured 1.4±0.1 m² /g.

EXAMPLE 6

A Bi-hydroxide-suspension was prepared in the same way as in example 5,after which 35.51 g of Cr-hydroxide (65.2% of Cr₂ O₃) were mixed in andthe mixture was processed in the same way as in Example 5. Grinding andworking up in the usual way gave orange mixed-phase pigments having thecomposition Bi₁.7 Cr₀.3 O₃ (5.30±0.2% of Cr₂ O₃ ; 94.6±0.25% of Bi₂ O₃)with BET-surfaces of 1.5±0.1 m² /g.

EXAMPLE 7

0.897 kg of CrO₃ (99%, Merck) were dissolved in 10 liters of water and13.150 kg of basic bismuth carbonate (90.1% of Bi₂ O₃,) were added inportions while stirring together with another 7.5 liters of water(30-liter stirrer-equipped vessel+Kotthoff mixing siren). The suspensionwas homogenized for 1 hour (pH-value ˜2.5; solids content <50%) and thendried on metal plates at 105° C. to a residual moisture content of lessthan 2%. Size reduction to powder form was carried out in a dry mill.Half the raw-material mixture was then calcined for 4 h at 750° C. in amuffle furnace (porcelain crucible) and the other half was reacted for 4hours at 750° C. in a rotary kiln.

The calcines were subjected to dry preliminary size reduction (15minutes in a mortar mill), wet-ground for 5 minutes in a sand mill,filtered, washed and dried at 105° C.

6.1 kg of an orange mixed-phase pigment having the composition Bi₁.7Cr₀.3 O₃ (5.4±0.1% of Cr₂ O₃ ; 94.5±0.1% of Bi₂ O₃) were obtained ineach case. The specific BET-surfaces measured 1.9±0.1 m² /g.

EXAMPLE 8

447.03 g of basic bismuth carbonate (88.6% of Bi₂ O₃) and 29.74 g ofchromium oxide filter refuse (77.9% of Cr₂ O₃) were intensively mixedwith 500 ml of water in a Kotthoff mixing siren (solids contentapproximately 48%) and adjusted to a pH-value of 2.5 with a little HNO₃.

After the paste had been dried (at 105° C. in a drying cabinet) andsize-reduced, the homogeneous powder obtained was calcined in air in arotating tube furnace at temperatures of 700° and 750° C., respectively(2 h).

The calcines were subjected to dry preliminary size reduction (15minutes in a mortar mill) and then wet-ground for 5 minutes in a sandmill, followed by filtration, washing and drying at 105° C.

The orange colored bismuth oxide/chromium oxide mixed phase pigments hadthe composition Bi₁.7 Cr₀.3 O₃ (5.35±0.1% of Cr₂ O₃ ; 94.4±0.1% of Bi₂O₃) and specific BET-surfaces of 1.8±0.1 m² /g.

EXAMPLE 9

202.25 g of metallic finely powdered bismuth were dissolved in 400 ml of64.5% HNO₃ in a 5-liter stirrer-equipped vessel and the resultingsolution was adjusted to a pH-value of 1.5 by the addition ofapproximately 1.5 liters of water at room temperature. 129.08 g ofCr(NO₃)₃.9H₂ O were introduced into the clear Bi-(III)-nitrate solutionand the pH-value of the solution was then increased to 10 by theaddition of 50% KOH at room temperature. A green suspension was formedand, after homogenization for 30 minutes, the product was filtered,washed (until free from nitrate), dried at 105° C. and size-reduced to ahomogeneous powder.

The raw-material mixture was divided into 3 equal parts which werecalcined for 1 hour in air in a muffle furnace at temperatures of 575°,650° and 725° C., respectively, subjected to dry preliminary sizereduction for 10 minutes and then wet-ground in a sand mill for 60minutes. The mixed phase pigments described in the following table wereobtained after filtration, washing and drying (composition Bi₁.5 Cr₀.5O₃ (9.70±0.1% of Cr₂ O₃, 90.0±0.1% of Bi₂ O₃):

    ______________________________________                                                                     BET-surface                                      Calcination and grinding                                                                      Color        m.sup.2 /g                                       ______________________________________                                         1 h at 575° C.                                                                        yellow       9.2                                              60 mins sand mill                                                              1 h at 650° C.       8.1                                              60 mins sand mill                                                              1 h at 725° C.                                                                        golden yellow                                                                              8.3                                              60 mins sand mill                                                             ______________________________________                                    

EXAMPLE 10

568,42 g of bismuth metal powder <40 μm 99.99% purity and 6 l of waterwere stirred with a Kotthoff mixing siren (2800 r.p.m.) in a 100 l glasstube, with the introduction of oxygen, for 4 h at about 70° C. 48.00 ofCrO₃ were added to the yellowish white suspension formed after 4 h andthe mixture was homogenised for a further 4 h with the Kotthoff mixingsiren at ˜70° C. The orange-brown raw material mixture was filtered,dried at 105° C. and pulverised in a mill.

150 g portions of this raw-material mixture were calcined in air in arotating tube furnace under the following conditions:

(a) 150 g of the mixture were heated from room temperature to 750° C.within 6 h and left at the final temperature for 3 h.

(b) 150 g of the mixture were introduced directly into the hot furnaceand left for 6 h at 750° C.

(c) 150 g of the mixture were introduced directly into the hot furnaceand left for 16 h at 750° C.

(d) 150 g of the mixture were introduced directly into the hot furnaceand left for 32 h at 750° C.

The calcination products were subjected to dry preliminary sizereduction (minutes in a mortar mill) wet-ground for 5 minutes in astirred-equipped ball mill at 1500 r.p.m., filtered, washed and dried at105° C.

A description of the products can be found in the following table:

    ______________________________________                                                                         BET-surface                                  Calcination conditions   Colour  m.sup.2 /g                                   ______________________________________                                        (a) from room temp. to 750° C.                                                                            1.9                                            3 h at 750° C.                                                     (b) directly in the hot furnace    1.8                                            6 h at 750° C.                                                     (c) directly in the hot fur- orange                                                                              1.6                                            nace, 16 h at 750° C.                                                                           red                                              (d) directly in the hot furnace,   1.6                                            32 h at 750° C.                                                    ______________________________________                                    

These mixed phases are shown by chemical analysis, to have the followingfollowing composition: Bi₁, 7Cr_(O), 3^(O) 3 with 4.5±0.15% Cr₂ O₃ and94.6±0.2% Bi₂ O₃.

EXAMPLE 11

71.06 g of bismuth metal powder (<40 μm, 99.99% purity) and 4.606 g ofCr₂ O₃ (99.5% purity) were dispersed in a stirrer-equipped ball millwith water for 3 h at 72° C. and 1500 r.p.m. with gassing using oxygen.The light green suspension was filtered, dried and calcined in air in arotating tube furnace for 6 h at 750° C.

Following calcination the batch was subjected to dry preliminary sizereduction (5 minutes in a mortar mill), wet-ground in a stirrer-equippedball mill (5 minutes at 1500 r.p.m.), filtered washed and dried.

The end product obtained was an orange-yellow mixed phase having thecomposition Bi₁, 7Cr_(O), 3^(O) 3 (5.3% Cr₂ O₃ ; 94.5% Bi₂ O₃) andspecific surface of 4 m² /g.

EXAMPLE 12

518.32 g of bismuth metal powder (<40 μm; 99.99% purity) were convertedinto bismuth hydroxide following the same procedure as in Example 10 and70.72 g CrO₃ (99%) were then added. After homogenising the mixture for 4hours in the same apparatus an orange-coloured suspension was formedwhich was filtered, washed and dried at 105°. The dried raw-materialmixture was divided into a powder and calcined as follows in air in arotating kiln:

(a) 300 g were calcined for 6 h at 600° C., having been, directlyintroduced into the hot furnace.

(b) 300 g were calcined for 6 h at 650° C., having been directlyintroduced into the hot furnace.

The calcination produces were subjected to dry preliminary sizereduction (5 minutes in a mortar mill), wet-ground in a stirrer-equippedball mill (5 mins. at 1500 r.p.m.), filtered, washed and dried at 105°C.

The mixed phases have the composition Bi, 55Cr_(O), 45^(O) 3 and can bedescribed as follows:

    ______________________________________                                                                        BET-Surface                                   Calcination conditions                                                                            colour      m.sup.2 /g                                    ______________________________________                                        (a)   6 h at 600° C.                                                                       yellow      3.4                                           (b)   6 h at 650° C.                                                                       golden yellow                                                                             2.5                                           ______________________________________                                    

EXAMPLE 13

796.11 g of Bi₂ O₃ (99.5%) and 60.60 g of CrO₃ (99.0%) were processedwith 250 ml of water in a Kotthoff mixing siren to form a homogeneouspaste (solids content approximately 75%, pH-value 6). After the rawmaterial mixture had been dried (105° C.) and size-reduced, the finepowder obtained was calcined in a rotating tube furnace (for 2 hours at750° C.). The calcine was ground for 5 minutes in a sand mill, filtered,washed and dried.

According to analysis, the brownish-orange end product had thecomposition Bi₁.7 Cr₀.3 O₃ (5.30% of Cr₂ O₃ ; 94.50% of Bi₂ O₃).

b. 796.11 g of Bi₂ O₃ (99.5%) and 58.56 g of chromium oxide paste (77.9%of Cr₂ O₃) were mixed, calcined and worked up in the same way as inExample a.

The end product was brown-orange in color with tinges of olive and hadthe composition Bi₁.7 Cr₀.3 O₃ (5.40% of Cr₂ O₃, 94.40% of Bi₂ O₃).

In both cases, the use of the Bi₂ O₃ led to products of inferior pigmentquality.

It will be appreciated that the instant specification and examples areset forth by way of illustration and not limitation, and that variousmodifications and changes may be made without departing from the spiritand scope of the present invention.

What is claimed is:
 1. A process for preparing a yellow to orange-redbismuth oxide/chromium oxide mixed phase according to claim 1,comprising intensively dispersing metallic bismuth in an aqueoussuspension to produce a compound of bismuth-(III), mixing the dispersionwith an appropriate quantity of a chromium-(III)- orchromium-(VI)-compound, calcining the mixture in air at a temperaturefrom about 500° to 800° C. and subsequently grinding.
 2. A processaccording to claim 1, wherein the bismuth-(III)-compound is at least oneof bismuth hydroxide and basic bismuth carbonate.
 3. A process accordingto claim 1, wherein the chromium compound is at least one ofchromium-(III)-oxide, chromium hydroxide, chromium-(VI)-oxide,chromium-(III)-acetate and chromium oxide hydroxide.
 4. A processaccording to claim 1, wherein calcination is carried out at atemperature from about 600° to 750° C.
 5. A process according to claim1, wherein calcination is carried out with or without circulation of theproduct in one step.
 6. A process according to claim 1, whereincalcination lasts between about 0.5 and 32 hours.
 7. A process accordingto claim 1, wherein the bismuth-(III)-compound is at least one ofbismuth hydroxide and basic bismuth carbonate, the chromium compound isat least one of chromium-(III)-oxide, chromium hydroxide,chromium-(VI)-oxide, chromium-(III)-acetate and chromium oxidehydroxide, and calcination is carried out for from about 2 to 16 hourswith circulation.